Large Core Optical Fibers for Medical Applications
By Joe Zhou, John H. Shannon, James P. Clarkin
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Summary:

Medical procedures using high power lasers to alter or vaporize body tissues are increasingly common. Popular lasers in these procedures include Nd:YAG, diode and Ho:YAG lasers. Large core hard polymer clad optical fiber (HPCF) is becoming the choice for fiber optic assemblies that are used to deliver laser energy. While proper fiber design, fiber termination and end face preparation are critical to the power performance of a fiber optic assembly, the type of laser source and the laser launch conditions both have an impact. For medical product development engineers, the key to project success is to develop a close relationship with an experienced fiber supplier in the early stage of product development. This not only shortens the development cycle but also saves time and reduces cost when the project moves into the production phase later.

Introduction

Medical procedures using high power lasers to alter or vaporize body tissues are increasingly common. Some examples include BHP or enlarged prostate surgery, laser lithotripsy or stone fragmentation, endovenous laser therapy, and laser angioplasty. Less common are laser trabeculotomy, TMR, and dental/oral surgeries.

Typical lasers used in the medical procedures are Nd:YAG lasers (1064nm), frequency doubled Nd:YAG lasers (532nm), diode lasers (800~850nm and ~980nm), and Ho:YAG lasers (2.1µm). For these common lasers, pure silica core step index multimode optical fiber can be a good choice as it transmits light well at visible and IR wavelengths up to 2.1µm. CO2 lasers at 10.6µm and Er:YAG laser at 2.9µm are also used for high power medical applications. However, silica core fiber does not work for these two lasers due to high attenuation. Instead, silica based hollow waveguides1 optimized at the relevant wavelengths can be employed. For excimer lasers emitting UV light at 308nm, optical fiber must be specially processed or treated to provide good and stable transmission. It is important to select a proper fiber for specific lasers at specific applications.

In medical procedures, laser energy is delivered through an optical fiber assembly. Use of optical fiber is beneficial to patients and medical doctors and also reduces medical expense. Since the fiber is thin and flexible, it can be easily and tightly bent. Therefore, only a small incision or cut is required to insert the fiber into the body and deliver the light energy to the target tissue. Thus, medical procedures using fiber optic delivery are minimally invasive. The patient recovers faster from the procedure. In practice, patients have shorter hospital stays, often an outpatient visit, which will result in medical cost reduction. As laser energy can be delivered through fiber to a target area without damaging the surrounding tissue, the procedure has high efficacy. Bleeding is reduced during procedures because of the small incision and coagulation characteristics of some lasers. The procedures provide patients a less traumatic experience as a result.

Optical Fiber

Pure silica core step index multimode optical fibers have several advantages over telecom fiber. First, the fiber is capable of handling high power. The large core diameter, often >0.1mm, enables the fiber to transmit more light. High numerical aperture (NA) gives the fiber a wider angle of light collection and thus enables more light to be coupled into the fiber. High core to clad ratio enables the fiber to transmit maximum light power for the same fiber core diameter while its flexibility is maintained. In addition, the fiber is capable of transmitting higher laser power because pure silica material has a higher melting temperature and damage threshold than doped silica. A second benefit is lower cost, with a minimal amount of expensive dopant materials used in a pure silica core fiber. Also, the fiber has high mechanical strength and flexibility. Fiber is proof tested in line up to 150kpsi and has a small bend radius. Other benefits include ease of termination and the ability to sterilize these fibers using standard methods including autoclave and ETO.

Silica fiber with polyimide, hard clad and silicone coating or buffer is common in medical applications. Polyimide works at temperatures up to 400°C. As this coating is tough and thin, polyimide coated fiber is a good option for bundles and often require no extra buffer or jacket in practice. Hard clad, an optical polymer with a lower refractive index than silica, can also be used as cladding material. It provides extra mechanical strength to the fiber, as well as facilitates quick cleaving and easy field termination. Silicone has a high temperature rating up to 200OC. A rubbery material, silicone is an excellent choice for applications requiring minimal micro-bending loss. Secondary coatings are typically extruded onto the fiber for additional mechanical protection. Common materials are Tefzel®, nylon and Telfon®. As a fluoro-polymer, Tefzel® and Telfon® are chemically inert while nylon is often used in cases where the jacket needs to be glued to the connector.

Hard polymer clad fiber (HPCF) is popular for medical applications. From a viewpoint of fiber configuration, there are single-clad HPCF and dual-clad HPCF. Examples of single clad and dual clad HPCFs are shown in Figure 1 and Figure 2, with example configurations listed in Table 1 and Table 2. Custom sizes and configurations are also available. Dual clad differs from single clad in that dual clad has an extra thin fluorine doped silica glass layer between the pure silica core and outer hard polymer coating. The additional hard polymer coating in dual clad HPCF acts as a secondary cladding so that additional light can be guided in the silica cladding. The dual clad HPCF has a higher power damage threshold and covers a broader wavelength range, up to 2.1µm in infra-red. However, the dual clad has lower NA (0.22NA versus 0.37 or 0.48NA for single clad) and costs moderately more than single clad. Single clad is generally recommended for an application before dual clad is considered.